perf script python: Add printdate function to SQL exporters

Introduce a printdate function to eliminate the repetitive use of
datetime.datetime.today() in the SQL exporting scripts.

Signed-off-by: Tony Jones <tonyj@suse.de>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Link: http://lkml.kernel.org/r/20190309000518.2438-5-tonyj@suse.de


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>